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Fig. 1. ErbB4 mutants are constitutively tyrosine phosphorylated. ErbB4 expression and tyrosine phosphorylation were assayed in PA317 cells infected with retroviruses that direct the expression of wild-type ErbB4 or the ErbB4 mutants. Cells infected with the LXSN recombinant retrovirus vector control served as the negative control. Lysates were prepared from each of the cell lines, and ErbB4 was immunoprecipitated from 1000 µg of each lysate. Samples were resolved by SDS-PAGE, electroblotted to nitrocellulose, and immunoblotted with an anti-phosphotyrosine antibody (left panel). The blot was then stripped and probed with an anti-ErbB4 rabbit polyclonal antibody (right panel). The band at the top of the blots represents ErbB4.